Theoretical Investigations of Intramolecular Energy Transfer Rates and Pathways for Vinyl Bromide on an ab Initio Potential-Energy Surface

Abstract

The dynamics of intramolecular energy transfer in vinyl bromide have been investigated using projection methods and results obtained from classical trajectories computed on a global potential-energy surface (PES1) that has been fitted to a large database of energies obtained from ab initio electronic structure calculations at the MP4 level of theory with extended basis sets and experimental thermochemical data. Similar calculations on two additional potential surfaces derived from PES1 are also reported. The intramolecular energy transfer dynamics obtained on these surfaces are compared with one another and with previously reported results using an empirical hybrid-type surface (EPS) fitted to thermochemical, structural, kinetic, and spectroscopic data and the results of a limited number of electronic structure calculations for the transition states. The temporal variation of the average vibrational mode energies are computed from the projected mode kinetic energy profiles using the equipartition theorem. Total energy decay rates and the pathways of energy flow for initial 3.0 eV excitation of each of the 12 vibrational modes in the equilibrium configuration have been determined. The results show that the total relaxation rate for each mode can be characterized with fair accuracy by a first-order rate law. The minimum decay rate among the 12 modes is found to be significantly larger than the decomposition rate for vinyl bromide at an internal energy of 6.44 eV. Such an inequality is a necessary condition to satisfy the basic assumption of statistical theories. However, it is also found that energy transfer is not globally rapid. Some modes are essentially inactive for over one picosecond. Consequently, it is possible that statistical theory will not adequately describe the decomposition processes occurring upon excitation of vinyl bromide. Both the computed energy-transfer pathways and the total mode relaxation rate coefficients are found to be very sensitive to small variations in the surface curvatures near equilibrium and along the reaction coordinates. It is concluded that IVR rates for systems as complex as vinyl bromide computed using trajectory methods and ab initio potentials based on extensive electronic structure calculations will probably yield results with the correct order of magnitude but with errors that can be as large as a factor of 3. The sensitivity of the results to the fine details of surface curvature suggests that if total mode relaxation rates can be accurately measured, they will provide a very demanding test of potential-surface accuracy.